The preparation of vinyl acetate by reaction of ethylene with acetic acid and oxygen or oxygen-containing gases in the gas phase over fixed-bed catalysts is already known. The reaction is generally carried out at pressures from 1 to 2.5 MPa and temperatures from 100° C. to 250° C. The reaction is typically conducted in the presence of a suitable catalyst, which may comprise palladium, an alkali metal acetate promoter, and optionally a co-promoter, e.g., gold or cadmium, on a catalyst support. One exemplary vinyl acetate production process, set forth in U.S. Pat. No. 6,696,596, uses a reaction in the gas phase with oxygen or oxygen containing gasses over fixed-bed catalysts. Another example is disclosed in U.S. Pat. No. 6,040,474, which describes the manufacture of acetic acid and/or vinyl acetate using two reaction zones wherein the first reaction zone comprises ethylene and/or ethane for oxidation to acetic acid with the second reaction zone comprising acetic acid and ethylene with the product streams being subsequently separated thereby producing vinyl acetate. Also, U.S. Pat. No. 6,476,261 describes an oxidation process for the production of alkenes and carboxylic acids such as ethylene and acetic acid, which are reacted to form vinyl acetate. Each of the references mentioned above is incorporated herein by reference in its entirety.
In the multistage catalytic process, vinyl acetate and water are formed in equimolar amounts. The total oxidation of ethylene, which cannot be entirely avoided, forms carbon dioxide and water. More than 1 mol of water per mole of vinyl acetate is thus obtained. In general, the weight of water is about one quarter of the weight of the vinyl acetate formed. Apart from carbon dioxide, small amounts of other by-products, including ethyl acetate, are formed in a proportion of 1000 to 2000 wppm, based on the vinyl acetate formed. Only a small amount, e.g., not more than 250 wppm, of ethyl acetate is generally acceptable in the pure vinyl acetate. The removal of ethyl acetate requires a large amount of energy. The prior art addresses various methods of reducing the energy consumption in the purification of vinyl acetate with removal of ethyl acetate and other by-products.
The mixture used for the reaction contains a molar excess of the stoichiometrically required amount of ethylene. Accordingly, the ethylene conversion is relatively low (about 10%) and, to maintain ethylene efficiency, the unreacted ethylene has to be recirculated to the reaction zone. Product vinyl acetate is usually then separated off from the mixture of gaseous reaction products in a multistage process.
U.S. Pat. No. 4,818,347 describes a process in which the hot gas mixture leaving the vinyl acetate reactor, namely ethylene, acetic acid, vinyl acetate, water, carbon dioxide, ethyl acetate, oxygen and inerts such as, for example, nitrogen and argon, is introduced into a first distillation column which operates without additional heating, known as the predewatering or predehydrating column. A gas mixture leaves the top of the predehydrating column and is brought into contact with a runback stream in a heat exchanger. The heat exchange results in the gas mixture being cooled and the runback stream being correspondingly heated. The gas mixture subsequently goes from the heat exchanger to a condenser. The material that is liquefied in the condenser is collected in a collection vessel, in which separation into an aqueous phase and an organic phase occurs. The aqueous phase may be discharged while all or part of the organic phase is recirculated as reflux to the top of the predehydrating column.
The material which has not been liquefied in the condenser may contain gaseous vinyl acetate. The gaseous vinyl acetate is scrubbed out of the gas mixture in a scrubbing column that uses acetic acid as scrubbing liquid, known as the circulating gas scrubber. The remaining tail gas is recirculated to the reactor. The bottom stream exiting the circulating gas scrubber and the remainder of the liquefied organic phase from the condensate of the predehydrating column (if any) are collected. In some cases all of the liquefied organic phase from the condensate is used as reflux to the predehydrating column and no liquefied organic phase remains.
The predehydrating column yields a bottom product comprising a mixture comprising vinyl acetate, acetic acid and about half of the water of reaction and also by-products. The other half of the water of reaction is separated without introduction of energy and forms the aqueous phase of the condensate formed on cooling of the vapor from the top of the predehydrating column, as discussed above.
The bottom product from the predehydrating column is firstly fed into a collection vessel, also referred to as the crude vinyl acetate collection vessel, and subsequently worked up in a second distillation column, known as an azeotrope column. Vinyl acetate saturated with water is obtained as overhead product. A side stream comprising ethyl acetate and a bottom product, which is recirculated to the system as recycle acetic acid, are also obtained. The side stream comprising ethyl acetate is discharged. The vinyl acetate saturated with water that is not returned as runback to the top of the second distillation column is combined with the outflow from the bottom of the circulating gas scrubber and the remainder of the liquefied organic phase from the condensate from the predehydrating column.
The mixture is subsequently fed to a third distillation column, known as the dewatering column. The vapor from the top of this column is, after condensation, virtually entirely recirculated as runback. A side offtake stream is withdrawn from the dewatering column and separated into an aqueous phase and an organic phase, with the aqueous phase then being discharged and the organic phase being returned to the column. A dry vinyl acetate/acetic acid mixture is taken off at the bottom of the dewatering column and fed to a fourth column, known as the pure vinyl acetate column. In this column, vinyl acetate, which is virtually free of ethyl acetate, is obtained as overhead product, while the bottoms from this column, which comprise acetic acid, high boilers and traces of vinyl acetate and ethyl acetate are recirculated to the process, after discharge of a substream.
Other separation systems include those described in U.S. Pat. Nos. 6,696,596; 6,476,261; 6,228,226; 6,040,474; 5,066,365; 3,905,875; 3,838,019; and 3,438,870, the entireties of which are incorporated herein by reference.
Finally, it is desirable to remove of water and ethyl acetate as early as possible in the work-up process so as to reduce the amount of these undesirable materials carried through the overall work-up process. In doing so, the associated energy-intensive removal in the pure vinyl acetate distillation column can be reduced or eliminated.
Thus, even in view of these references, the need exists for an efficient process for producing vinyl acetate.